Hari Krishnan R, SRM Institute of Science and Technology
An Epithelial-Mesenchymal transition is a reversible event that occurs in embryonic development and cancer. As indicated by the name, this process involves the conversion of epithelial cells to mesenchymal cells. Cells undergoing this process are highly capable of migration, proliferation, and differentiation into various cell types. The epithelial cells having an EMT phenotype lose their cell polarity and cell adhesion capability and acquire mesenchymal traits. Mesenchymal cells produced through EMT are essential for tissue regeneration, wound healing but also leads to tumour invasiveness and metastasis.
Types of EMT
EMT is broadly classified into 3 types based on their role. Type I is associated with embryo development and gastrulation, Type II is associated with wound healing, tissue regeneration, and organ fibrosis and, Type III is associated with cancer malignancies involving invasion and metastasis.
I. EMT in Embryonic Development- Type I
Epithelial-Mesenchymal transition is essential during gastrulation as it mediates the differentiation of embryonic stem cells to their final cell type. It is known to play a major role during neural crest formation, myogenesis, secondary palate formation, and heart valve development. In embryonic development, EMT in embryonic development is divided into three stages- Primary EMT, secondary EMT, tertiary EMT.
Primary EMT occurs even before the implantation and aids in the formation of the parietal endoderm. The next EMT event occurs after implantation and helps in the formation of mesoderm from the primitive endoderm (gastrulation). At this stage, the epithelial cells lose their adhesion properties and gain migratory properties of the mesenchymal cells.
Secondary EMT is a dynamic process that focuses particularly on differentiation. The mesenchymal cells generated in this step are highly capable of differentiation into various cell lineages. Once after the differentiation, these cells undergo a reverse process of EMT, known as MET (Mesenchymal-Epithelial Transition) through which they condense into epithelial cells thereby forming the notochord.
The tertiary EMT is a complex event cyclically involving three rounds of EMT-MET ultimately leading to the formation of the embryonic heart. At the end of the first cycle, a two-layer epithelium is formed followed by cardiac mesoderm layers in the second cycle and atrioventricular septum in the third cycle. Finally, the heart valves are formed from completely differentiated tissues.
II. EMT in Wound Healing, Tissue regeneration- Type II
Tissue repairing events mainly involve cells that express EMT phenotype. During wound healing, keratinocytes surrounding the wound undergo EMT to differentiate into fibroblast cells that contribute to wound closing. Snail2 is an important factor in this context as its activation promotes cell migration and differentiation. In women, EMT is activated in postovulatory cells of the ovary to repair the tissue damage after every menstrual cycle.
Disruption of the normal tissue/organ architecture leads to fibrosis. This happens due to recurrent damage and inflammation to the tissues and, it is being found that this process is mediated by EMT. Epithelial cells of such tissues or organs are converted to specialized cells like myofibroblasts through EMT, which causes overproduction of collagen compromising the organ function.
III. EMT in Cancer metastasis- Type III
EMT in adult tissues is most commonly associated with the initiation of metastasis, during which the epithelial cells/carcinoma gain biochemical and structural properties of mesenchymal cells. These mesenchymal cells have a unique property of intravasation, migration, immune evasion, and apoptosis resistance. The loss of an epithelial cell marker E-cadherin is the signature step in EMT, which prevents the formation of strong adherent junctions between adjacent epithelial cells and thereby lose their integrity and move freely in the ECM. The activation of the TGF-β pathway is responsible for cytoskeleton reorganization and thereby leading to loss of epithelial cell polarity.
The cancer cells undergoing EMT is released into the bloodstream where it is bound by the platelets. Platelets have two roles in this context- 1. Help in immune evasion and 2. Binds to the P-selectin on the blood vessel wall in the target site which facilitates the exit of cancer cells from the bloodstream to form secondary tumors.
Biomarkers of EMT-
The biomarkers of EMT are broadly classified based on their location in the cell. These are cell surface proteins, ECM proteins, transcription factors and, cytoskeletal markers (Table 1). These biomarkers demonstrate all 3 types of EMT.
1. Cell surface markers
Loss of E-cadherin is a fundamental regulator of the EMT process making it an important biomarker. E-cadherin is a cell adhesion molecule widely expressed in epithelial cells and during EMT the levels of E-cadherin go down. Recent studies have identified that during the EMT process E-cadherins are converted to N-cadherins. N-cadherins are a potential marker of mesenchymal cells. This switching from E-cadherin to N-cadherin is called cadherin switch and it has helped scientists monitor EMT.
Integrins are transmembrane receptors that act as cell-ECM adhesion molecules. Integrins are generally found in both epithelial and mesenchymal cells. Two integrin molecules, β6 and α5 are potential biomarkers of EMT. A high level of β6 integrin has been identified in type-III EMT of colon cancer. The α5 integrin is found in all 3 types of EMT. In type-I EMT de novo expression of α5β1 is observed, which acts as a fibronectin receptor. In type-II EMT, increased α5 is associated with kidney fibrosis. Finally, in type-III EMT, increased α5 is associated with metastasis of melanoma.
2. Extracellular matrix proteins
Fibronectin is a glycoprotein and a principal component of the ECM. Due to its high molecular weight and dense nature, it acts as a scaffold for fibrillar ECM. During gastrulation, fibrillar ECM is one of the first components to form thereby making fibronectin an important biomarker of type-I EMT. Laminin, a component of the basement membrane is a well-established biomarker of EMT. Type I and type II EMT are associated with loss of laminin1 (α1β1γ1). In contrast, laminin5 (α3β3γ2) upregulation is associated with Type II and type III EMT in tissue fibrosis and cancer, respectively
3. Transcription Factors
The Snail family of proteins are suppressors of E-cadherin expression thereby making it a potential biomarker of all 3 types of EMT. Snail1 and Snail2 are associated with reduced expression of epithelial markers, increased expression of mesenchymal markers, suppression of apoptosis, inhibition of cell division by acting on cyclin-dependent kinases (eg-CDK4).
Forkhead Box C2 (FOXC2) transcription factor is widely expressed in metastatic breast cancers and also plays a role in angiogenesis and organogenesis. FOXC2 is a pleiotropic inducer of type-III EMT.
The helix-loop-helix protein Twist is associated with the regulation of cell differentiation. A high level of Twist expression is associated with cancers, tissue fibrosis, and gastrulation. Twist upregulates the expression of N-cadherin and fibronectin in type-III metastatic cancers.
The nuclear riboprotein CBF-A is observed in type-II and type-III EMT associated with organ fibrosis and metastasis, respectively
4. Cytoskeletal markers-
Calcium-binding protein FSP1 is a fibroblast EMT biomarker in cancer and fibrosis. In type-II EMT, FSP1 is expressed during the transition to fibroblasts whereas, in type-III EMT, FSP1 is expressed in metastatic breast cancer cells.
Cytoskeletal intermediate filament vimentin is a biomarker of type-I and type-III EMT. Vimentin is expressed first during gastrulation making it a potential marker for type-I EMT. In type-III EMT, vimentin expression is related to increased invasiveness and metastasis.
β-catenin is a marker of all three types of EMT. The two functions of β-catenin during EMT are linking cadherin to the cytoskeleton and forming a coactivator complex with T-cell factor (TCF). The β-catenin/TCF complex controls the expression of Snail1 involved in EMT. The actin family member α-SMA is usually expressed in myoepithelial cells and is a marker of type-II and type-III EMT. In type-II, it is associated with cardiac fibrosis whereas, in type-III, α-SMA is confined to basal-type breast cancer.
Conclusion
EMT is a key process in cancer and fibrosis, and although its mechanisms are well characterized, there are still many challenges that need to be addressed. The expression of markers of EMT usually depends upon the type of cells and the signaling pathway activated. This makes it difficult to understand their degree of progression. It is also difficult to observe the nature of EMT in-vivo due to its reversible nature, but recent advances in imaging and cell culture systems have opened new options which may enable us in targeting EMT in various diseases.
Also read: Oral vitamin D supplementation helps prevent colorectal cancer
References-
- Kim, D. H., Xing, T., Yang, Z., Dudek, R., Lu, Q., & Chen, Y. H. (2017). Epithelial Mesenchymal Transition in Embryonic Development, Tissue Repair and Cancer: A Comprehensive Overview. Journal of clinical medicine, 7(1), 1. https://doi.org/10.3390/jcm7010001
- Roche J. (2018). The Epithelial-to-Mesenchymal Transition in Cancer. Cancers, 10(2), 52. https://doi.org/10.3390/cancers10020052
- Lamouille, S., Xu, J., & Derynck, R. (2014). Molecular mechanisms of epithelial-mesenchymal transition. Nature reviews. Molecular cell biology, 15(3), 178–196. https://doi.org/10.1038/nrm3758
- Zeisberg, M., & Neilson, E. G. (2009). Biomarkers for epithelial-mesenchymal transitions. The Journal of clinical investigation, 119(6), 1429–1437. https://doi.org/10.1172/JCI36183
Author- Hari Krishnan is a research enthusiast in Cell biology and Cell signalling. He is currently a final year Biotechnology Engineering student. He is a prolific Scientific writer with vast knowledge in diverse backgrounds of biotechnology. He is constantly focused on improving his knowledge and laboratory skills through various internships. A Chennai native, Hari is a spicy food lover and passionate about music and sports.
Author’s Publications:
- Krishnan, R. H. (2021). Comparison of the Drug Entrapment Efficiency of Almond Gum (Badam Pisin) to Span-60 Niosomes by Folin-Ciocalteu Assay. bioRxiv.
- Krishnan, H. (2020). TELOMERASE TARGETING IN CANCER. International Research Journal of Modernization in Engineering Technology and Science. https://doi.org/10.3390/cancers12082260
Hari’s LinkedIn – https://www.linkedin.com/in/hari-krishnan-r-6bbb841aa/
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